Selective Separation of Rare Earth Metals by Integrated Extraction and Crystallization

ABSTRACT

An integrated recovery process for rare earth recovery includes bringing the acid solution containing rare earth elements into contact with an organic phase containing an extraction agent to extract the rare earth elements into the organic phase. The organic phase is separated from the aqueous phase. The rare earth elements are either collected from the aqueous phase of the extraction process or the rare earth elements are stripped from the organic phase to an aqueous solution. Rare earth powders are collected from the aqueous solution by partly removing the aqueous solvent. Alternatively, the aqueous solvent can be completely removed, the collected rare earth powders are dissolved in an acidic solution, and the mixture is equilibrated for crystallization of the target rare earth element. In both cases, the crystals and a mother liquor are separated by solid/liquid separation.

RELATED APPLICATION(S)

The present Patent Application claims priority to U.S. Provisional Patent Application No. 61/956,022 filed May 30, 2013, which is assigned to the assignee hereof and filed by the inventors hereof and which is incorporated by reference herein.

BACKGROUND

1. Field

The present disclosure relates to the selective separation of rare earth elements from an acidic solution containing them by an integrated multi-component multi-stage reactive extraction and extractive crystallization process to facilitate the recovery of rare earth metals.

2. Background

The terms “rare earths” and “rare earth elements” are used to designate the group of elements between lanthanum, atomic number 57, and lutetium, atomic number 71, inclusive. To these elements should be added yttrium, atomic number 39, which is nearly identical with the rare earths in properties and usually can be found together with them in natural deposits.

Rare earth metals are used in a large number of high-tech applications. While China has 30% of the world reserves, nearly all these metals and oxides come from China today. At present, rare earth metals mainly are separated by solvent extraction. Despite the advances in extractants (such as P204 and P507) and process design (such as countercurrent extraction, third outlet and one-step scale-up), solvent extraction generates substantial waste.

SUMMARY

An integrated recovery process for rare earth recovery uses an acid solution for extracting rare earth elements. The acid solution containing rare earth elements is prepared, and brought into contact with an organic phase containing an extraction agent, thereby extracting the rare earth elements into the organic phase. The organic phase is separated from the aqueous phase, and the rare earth elements are collected from the aqueous phase of the extraction process, or the rare earth elements are stripped from the organic phase to an aqueous solution. Rare earth powders are collected from the aqueous solution by partly removing the aqueous solvent for rare earth powders to concentrate. Alternatively, the aqueous solvent can be completely removed, the collected rare earth powders are dissolved in an acidic solution, and the mixture is equilibrated for crystallization of a target rare earth element. In both cases, separation of the crystals and a mother liquor is performed by solid/liquid separation.

BRIEF DESCRIPTION OF THE DRAWING(S)

FIG. 1 is a diagram showing an integrated reactive extraction and extractive crystallization process.

DETAILED DESCRIPTION

Overview

To ameliorate the problem or excess waste generation, an integrated reactive extraction and extractive crystallization process is proposed. By crystallizing out the desired rare earth element(s) selectively along the existing extraction train, the amount of chemicals and solvents, as compared with extraction alone, can be greatly reduced.

Rare earth metals such as lanthanum, yttrium, cerium, neodymium, samarium and the like are widely used as a component of ceramic material, hydrogen absorbent, alloy component, catalyst component, or the like. Usually these metals can be found in a mingled state, and much effort has to be made to separate them into pure components. Conventional liquid/liquid extraction process is generally not ideal in the sense that it re-distributes the rare earth elements into the extract and raffinate, as it takes multiple steps and excessive solvent to produce pure rare earths.

Crystallization is introduced to the selective separation process of rare earth metals. A crystallization step can potentially yield a single element, bypassing the need to use a cascade of extraction tanks to get to the target element. In other words, the amount of chemicals and extractants can be reduced by precipitating out the target elements by crystallization.

Technique

Rare earths can be generally classified into three groups: light rare earths (La, Ce, Pr, Nd), middle rare earths (Sm, Eu, Gd) and heavy rare earths (Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc, Y). The present disclosure describes a technique for selective recovery of target rare earth elements by firstly separating light rare earths from middle and heavy rare earths with preliminary extraction processes. The target rare earth elements are then recovered by extractive crystallization. The technique provides an integrated process for selective recovery of rare earth elements with reduced chemical and solvent consumption. By way of non-limiting example, the technique can be performed according to the following sequence:

(1) Preparing an aqueous acid solution containing rare earths.

(2) Bringing the acid solution containing rare earths into contact with a substantially water-immiscible liquid organic solvent comprising an organic extractant to extract the rare earth elements into the organic phase. This establishes equilibration between the acid solution containing rare earths and a substantially water-immiscible liquid organic solvent comprising an organic extractant.

(3) Separating the organic phase containing extract from the residual acidic aqueous phase.

(4) Collecting the rare earth elements from the aqueous phase or stripping the rare earth elements from the organic extract to an aqueous solution.

(5) Removing part of the solvent from the aqueous solution, thereby allowing the rare earth elements to concentrate and crystallize.

(6) Separating the crystals and mother liquor by solid/liquid separation.

(7) Washing the crystallized solids to remove impurities and drying the washed solids in a vacuum oven.

Alternatively, the sequence can be performed by performing the sequence of parts (1)-(4), followed by:

(5) Collecting the rare earth powders by removing the aqueous solvent from the aqueous solution.

(6) Dissolving the collected rare earth powders in an acid solution.

(7) Equilibrating the mixture for crystallization of a target rare earth element.

(8) Separating the crystals and mother liquor by solid/liquid separation.

(9) Washing the crystallized solids to remove impurities and drying the washed solids in a vacuum oven.

The acidic solution may be any acid capable of dissolving the collected rare earth powders. Non-limiting examples are sulfuric acid (H₂SO₄), hydrochloric acid (HCl), nitric acid HNO₃, oxalic acid (H₂C₂O₄) and phosphoric acid (H₃PO₄).

The preparation of an aqueous acid solution containing rare earths is performed by dissolving rare earths in an acid solution with thorough mixing, having a rare earths concentration varying from 50 g/L to 150 g/L; alternatively, from 80 g/L to 120 g/L. The pH value of the aqueous acid solution is adjusted to a range of 0-3.

Bringing the acid solution containing rare earths into contact with a substantially water-immiscible liquid organic solvent containing an organic extractant is a non-limiting example of a liquid/liquid extraction. This liquid/liquid extraction is carried out by bringing the acid solution containing rare earths into contact with a substantially water-immiscible liquid organic solvent comprising an organic compound as an extractant. Non-limiting examples of water-immiscible liquid organic extraction agent include hexane, heptane, cyclohexane, kerosene, benzene and 1,2-dichloroethylene. Kerosene is used in an exemplary embodiment, and other solvents may be used. The criteria for the solvent includes a property of being immiscible with aqueous solvent. In some applications, it is desired that the solvent have a low evaporation rate. The evaporation rate is selected to be sufficiently low that the solvent remaining before drying will remain so that evaporation will not significantly affect the extractant concentration. Thus, the low evaporation rate is provided to make sure that evaporation of solvent will not affect the extractant concentration.

Examples of organic compounds include without limitation di(2-ethylhexyl) phosphoric acid (P-204), 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (P507), di-n-octylphosphinic acid, bis(2,4,4-trimethylpentyl)phosphinic acid, 2,4,4-trimethylpentylcyclohexylphosphinic acid, and dicyclohexylphosphinic acid. P-204 and P-507 are used in exemplary embodiments. The concentration of extractant ranges from 0.5M to 2M; alternatively, from 1 M to 1.5 M. The acid solution and the organic extraction agent are mixed in a ratio of 1:1 and allowed for equilibrating with gentle mixing for 1 hour at room condition.

The separation of the organic extract from the residual acidic aqueous phase may, by way of non-limiting example, be performed after equilibration by a separating funnel.

Light rare earth elements are collected from the aqueous phase of the extraction process. Middle and heavy rare earth elements are extracted to the organic extract and they are stripped back to an aqueous solution in the presence of 3M-5M hydrochloric acid. The mixing ratio of organic extract and aqueous solution is 1:1. Stripping is conducted for 1 hour with gentle agitation at room temperature.

The aqueous and organic phases are separated by a separating funnel after stripping.

Rare earth mixtures can be further purified by multiple steps of liquid/liquid extraction. After a series of extraction processes, the collection of the rare earth powders is then performed, by first removing the aqueous solvent. Rare earth powders can be collected by removing the aqueous solvents, thereby leaving the rare earth elements in dry powder form. The drying is done by means of rotary evaporation at a temperature of 60 to 80° C. and at a pressure of 60 to 70 mbar.

The dissolving of the collected rare earth powders in an acid solution may, by way of non-limiting example using sulfuric acid, be performed by using a sulfuric acid solution with a pH value of −0.1. The powders concentration ranges from 0.05 g/mL to 0.15 g/mL. The rare earth materials are then recovered.

In one non-limiting example, part of the solvent is removed and the solids crystallized.

Alternatively, a complete dissolution of solids can be obtained by cooling of the mixture. The mixture is cooled down to a temperature of 0° C. to 10° C. for complete dissolution of solids.

The equilibration of the mixture for crystallization of target rare earth element includes heating the solution to a temperature of 30° C. to 70° C., more preferably, a temperature of 40° C. to 60° C. for powder recrystallization. The solution is kept at the temperature with gentle stirring for a duration of 24 to 48 hours for attaining equilibrium.

The separation of the crystals and mother liquor by solid/liquid separation may, by way of non-limiting example, be performed with a separating funnel.

The washing of the crystallized solids to remove impurities and drying the washed solids in a vacuum oven may, by way of non-limiting example, be performed with an organic solvent like methanol, ethanol, hexane, heptane, etc. More preferably, they are washed with heptane to remove any impurities. The washed solids are finally dried in a vacuum oven.

EXAMPLES

Two examples are given below to demonstrate the method in the present disclosure for selectively recovery of target rare earth elements by multi-component multi-stage extractive reaction and crystallization process (FIG. 1).

An experiment was conducted with the partially beneficiated ore from JiangXi (JiangXi Rare Earths, JXRE produced by Jiangxi Yuean Superfine Metal Co., Ltd.) as the feed. 24.0054 g of JXRE was first dissolved in 289.5 mL of distilled water and 10.5 mL of sulfuric acid (95%, Acros) to give an aqueous phase with a pH value of 0.5. In the extraction stages 1 to 3, 95.14 g of di(2-ethylhexyl) phosphoric acid (P-204) was added to 230.7 g of kerosene (Fisher) to form the organic phase. At each stage of extraction, equal volumes of aqueous and organic phases were mixed together with gentle agitation for 1 hour at room condition. The phases were then separated by a separating funnel after they are brought into contact, in which the aqueous phase contacts with the organic phase. The extraction was conducted three times in a series (E1 to E3). The middle and heavy REs were extracted to the organic phases. The aqueous phase collected from E3 was rich in the light REs (La—Nd) and it was subjected to further extraction steps to separate La/Ce—Nd.

For extraction stages 4 to 6 (E4-E6), the organic phase was prepared by adding 304.98 g of 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (P507) to 119.54 g of kerosene. The pH value of the aqueous phase collected from E3 was adjusted to 3 by ammonium hydroxide (28%, Wako). The two phases were mixed in a ratio of 1:1 and allowed to come into contact with each other by means of gentle mixing for 1 hour at room condition. The two phases are brought into contact with each other, by the aqueous phase contacting the organic phase. The phases were then separated by a separating funnel after. The extraction was conducted three times in a series (E4 to E6).

Example 1 Recovery of La

The solvent of the aqueous phase collected from E6 was evaporated by rotary evaporation to obtain dried powders; of which, the purity of La was 0.73. 8.0647 g of dried solids were dissolved in 67.1953 g of sulfuric acid solution (pH=−0.1). The mixture was cooled down to 2° C. for complete dissolution of solids. The solution was then heated to 50° C. and kept for 48 hours for attaining equilibrium. Solid/liquid separation was carried out to separate crystals and mother liquor. The crystallized solids were washed with heptane and then dried in a vacuum oven. The purity of La of crystallized solids was 0.85.

Example 2 Recovery of Y

The middle and heavy REs in the organic phases collected from E1 to E3 were stripped to an aqueous solution by 3M hydrochloric solution (37%, Acros). Dried powders were collected by removing the aqueous solvent with a rotary evaporator; of which, the purity of Y was 0.98. 0.726 g of dried solids were dissolved in 2.2511 g of sulfuric acid solution (pH=−0.1). The mixture was cooled down to 2° C. for complete dissolution of solids. The solution was then heated to 50° C. and kept for 48 hours for attaining equilibrium. Solid/liquid separation was then carried out to separate crystals and mother liquor. The crystallized solids were washed with heptane and then dried in a vacuum oven. The purity of Y of crystallized solids was 0.99.

CONCLUSION

It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described and illustrated to explain the nature of the subject matter, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. 

1. An integrated recovery process for rare earth recovery, the process comprising: preparing an acid solution containing rare earth elements; bringing the acid solution containing rare earth elements into contact with an organic phase containing an extraction agent, thereby extracting the rare earth elements into the organic phase; separating the organic phase from the aqueous phase; collecting the rare earth elements from the aqueous phase of the extraction process or stripping the rare earth elements from the organic phase to an aqueous solution; collecting rare earth powders from the aqueous solution by removing the aqueous solvent, the collected rare earth powders comprising the rare earth elements; dissolving the collected rare earth powders in an acidic solution; equilibrating the mixture for crystallization of a target rare earth element; and separating the crystals and a mother liquor by solid/liquid separation.
 2. The integrated recovery process of claim 1, further comprising: washing the crystallized solids to remove impurities and drying the washed solids in a vacuum oven.
 3. The integrated recovery process of claim 1, further comprising: selecting, as the aqueous solution for extraction, a material selected from the group consisting of sulfuric acid (H₂SO₄), hydrochloric acid (HCl), nitric acid HNO₃, oxalic acid (H₂C₂O₄) and phosphoric acid (H₃PO₄).
 4. The integrated recovery process according to claim 1, wherein the organic phase comprises a hydrocarbon solvent.
 5. The integrated recovery process according to claim 1, further comprising: selecting, as the organic phase comprises a material, an organic solvent immiscible with the acid solution, and having a predetermined low evaporation rate.
 6. The integrated recovery process according to claim 1, further comprising: selecting, as the organic phase comprises a material, a material selected from the group consisting of hexane, heptane, cyclohexane, kerosene, benzene and 1,2-dichloroethylene.
 7. The integrated recovery process according to claim 1, further comprising: selecting, as the extracting agent, a material selected from the group consisting of di(2-ethylhexyl) phosphoric acid (P-204), 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (P507), di-n-octylphosphinic acid, bis(2,4,4-trimethylpentyl)phosphinic acid, 2,4,4-trimethylpentylcyclohexylphosphinic acid, and dicyclohexylphosphinic acid.
 8. The integrated recovery process according to claim 1, further comprising: prior to crystallizing the solids, extracting part of the solvent.
 9. The integrated recovery process according to claim 1, further comprising: selecting, as the aqueous solution for dissolving the collected rare earth powders, a material selected from the group consisting of hydrochloric acid, sulphuric acid, or nitric acid.
 10. An integrated recovery process for rare earth recovery comprising the following steps: (a) preparing an acid solution containing rare earth elements; (b) bringing the acid solution containing rare earth elements into contact with an organic phase containing an extraction agent, and thereby extracting the rare earth elements into the organic phase; (c) separating the organic phase from the aqueous phase; (d) collecting the rare earth elements from the aqueous phase of the extraction process or stripping the rare earth elements from the organic phase to an aqueous solution; (e) collecting rare earth powders from the aqueous solution by removing the aqueous solvent by partly removing the aqueous solvent to concentrate the mixture; and (f) separating the crystals and mother liquor by solid/liquid separation.
 11. The integrated recovery process of claim 10, further comprising: washing the crystallized solids to remove impurities and drying the washed solids in a vacuum oven.
 12. The integrated recovery process of claim 10, further comprising: selecting, as the aqueous solution for extraction, a material selected from the group consisting of sulfuric acid (H₂SO₄), hydrochloric acid (HCl), nitric acid HNO₃, oxalic acid (H₂C₂O₄) and phosphoric acid (H₃PO₄).
 13. The integrated recovery process according to claim 10, wherein the organic phase comprises a hydrocarbon solvent.
 14. The integrated recovery process according to claim 10, further comprising: selecting, as the organic phase comprises a material, a material selected from the group consisting of hexane, heptane, cyclohexane, kerosene, benzene and 1,2-dichloroethylene.
 15. The integrated recovery process according to claim 10, further comprising: selecting, as the extracting agent, a material selected from the group consisting of di(2-ethylhexyl) phosphoric acid (P-204), 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (P507), di-n-octylphosphinic acid, bis(2,4,4-trimethylpentyl)phosphinic acid, 2,4,4-trimethylpentylcyclohexylphosphinic acid, and dicyclohexylphosphinic acid.
 16. An integrated recovery process for rare earth recovery comprising the following steps: (a) preparing an acid solution containing rare earth elements; (b) bringing the acid solution containing rare earth elements into contact with an organic phase containing an extraction agent, and thereby extracting the rare earth elements into the organic phase; (c) separating the organic phase from the aqueous phase; (d) collecting the rare earth elements from the aqueous phase of the extraction process or stripping the rare earth elements from the organic phase to an aqueous solution; (e) collecting rare earth powders from the aqueous solution by removing the aqueous solvent, the collected rare earth powders comprising the rare earth elements;; (f) dissolving the collected rare earth powders in an acidic solution; (g) extracting at least a portion of the solvent; (h) equilibrating the mixture for crystallization of a target rare earth element; and (i) separating the crystals and mother liquor by solid/liquid separation.
 17. The integrated recovery process according to claim 16, further comprising: selecting, as the aqueous solution for dissolving the collected rare earth powders, a material selected from the group consisting of hydrochloric acid, sulphuric acid, or nitric acid.
 18. The integrated recovery process according to claim 16, wherein the organic phase comprises a hydrocarbon solvent.
 19. The integrated recovery process according to claim 16, further comprising: selecting, as the organic phase comprises a material, a material selected from the group consisting of hexane, heptane, cyclohexane, kerosene, benzene and 1,2-dichloroethylene.
 20. The integrated recovery process according to claim 17, further comprising: selecting, as the extracting agent, a material selected from the group consisting of di(2-ethylhexyl) phosphoric acid (P-204), 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (P507), di-n-octylphosphinic acid, bis(2,4,4-trimethylpentyl)phosphinic acid, 2,4,4-trimethylpentylcyclohexylphosphinic acid, and dicyclohexylphosphinic acid. 